It can be lethal, it makes patients ill for weeks and there’s no vaccine against it. Cases of dengue fever, whose symptoms usually include high temperature, body ache and fatigue, have increased 30-fold in the last 50 years. The World Health Organization estimates that around 50 to 100 million people are infected yearly and 2.5 billion people live in risk areas. After a successful trial run in Australia, a promising development that uses a common bacteria to fight dengue is about to be tried in one of the most affected countries in the world – Brazil.